Method of evaporating aqueous hydrochloric acid



Patented Feb. 18, 1936 Parser orricgs METHOD OF EVAPORATING mucousmnocncomo ACID Klaus Raschig, Ludwigshafen-on-the-Rhine,Ger-

many, sssixnor to Dr. F. Raschlg G. m.

b. IL,

.Ludwigshafen-on-the-lthine, Germany No Drawing.

Application November 22, 1934,

$393M No. 754,280. In Germany November z'l.

9 Claims.

oration vessels for aqueous hydrochloric acid from metals which areresistant against boiling hydrochloric acid and for this tungsten,tantalum. antimony alloys and the like have been used. Apparatus madefrom such materials are very expensive, however, on the one hand becauseof the high price 01 the metals mentioned and on the other hand onaccount of the dimculty in t sorlring them, which is to be ascribed totheir very high melting point and to their great brittleness. Apparatusof tungsten, tantalum and so forth are therefore used in practiceonly'to a limited extent, and the object of the present invention is tomake possible the evaporation oi aqueous hydrochloric acid inevaporation vessels made of relatively cheap and easily workable metals.

In order to overcome the difflculties which the evaporation of aqueoushydrochloric acid presents, it has also been proposed to carry out theevaporation in apparatus made of iron by introducing aqueoushydrochloric acid in a state of very iine sub-division into a stream ofsupermeans or atomizing nozzles and the like. Normal evaporatingapparatus in which the heat supplied is transferred directly to thematerial to he evaporated, cannot be used, however, for carryin 45 outthis process, but rather, special apparatus is example with steam, largeheat transfer surfaces heated by lire gases are employed. Moreover spe-019.! fans are required for moving the superheated hydrochloric acidvapour. Apart from these disadvantages, the use of direct firing whichis required for heating the heat transfer surfaces, demands constantsupervision of the apparatus by a special stud which renders the processexpensive. Moreover direct firing cannot news- It has thereiore beenproposed to make evap-.

heated hydrochloric acid vapour for example by necessary in whichinstead of direct heating for (cl. 23-154) I g sarily be carried out inevery i'actory on account of the danger of tire associated with it.

By reason of the foregoing remarks it is to be seen that an apparatussuitable for evaporating aqueous hydrochloric acid must fulfil the fol-5 lowing conditions: it must on the one hand be made of a technicallyeasily accessible and workable metal and it must moreover be of normalkind of construction and so must be capable of being heated with steam.

In a very surprising manner it has been found that copper, of which itis known that it does not resist attack by aqueous hydrochloric acid,can be employed in a most satisfactory manner as a material forevaporating vessels for hydrc- ,15 chloric acid, if the evaporation oi.the aqueous hydrochloric acid from such reaction vessels is carried outin the presence of phosphoric acid.

In this connection it has been round that even a small addition ofphosphoric acid to ti"- aqueous c hydrochloric acid considerably red?"-have attack of boiling hydrochloric acid N1... the other hand copperitself'in the presence of highly concentrated phosphoric acid at isciiugtemperature is not perceptlbly attacked hydrochloric acid. 25 The causeof this protective action exerted by the phosphoric acid is not known.Possibly it may be associated with the occurrence of a black coatingfirmly adhering to the copper wall which forms after a short period ofoperation.

It has previously been proposed to mix aqueous hydrochloric acid withanauxiliary liquid such as sulphuric acid, calcium chloride, magnesiumchloride or zinc chloride wlutions and the like and to evaporate it outof these solutions. In this process however, copper was not used as amaterial for the apparatus which is to be explained by the fact thatnone of these liquids is capable of preventing the attack of aqueoushydrochloric acid on copper.

By reason of the entirely unexpected fact that copper in the presence ofconcentrated phosphoric acid behaves quite differently with respect toaqueous hydrochloric acid from the way it behaves in the absence ofphosphoric acid, it is now possibleto manufacture evaporating apparatusfor aqueous hydrochloric acid solutions from an easily accessible andconveniently workable material. In particular it is now possible toconstruct such apparatus in the iorm usual for evaporation plant and soto employ the usual kind of heating such for example as steam heating,by means of normal heating tubes or heating coils. In this connectionalso the good heat conductivity of copper acts favourably. It is ascbviously also possible to employ other kinds of heating such forexample as electrical heating or the like.

Since, when using large evaporating apparatus, sometimes on account ofinsufficient heating, a condensation of aqueous hydrochloric acid maytake place in the upper parts of the apparatus which are not covered byliquid and this might very easilylead to a corrosion of the parts of theapparatus consisting of copper which are not subject to the protectingaction of the phosphoric acid, it is recommended when using largeevaporating apparatus that those parts of the apparatus which are notwashed over by the phosphoric acid should be heated by means ofadditional heating to such a temperature that no condensation of aqueoushydrochloric acid can take place. Hydrochloric acid vapour does notindeed perceptibly attack copper. The heating is advantageously efiectedby heating the upper part of the evaporating apparatus by means of steamtubes welded upon or fitted on, which tubes are well heat insulated onthe outside. Preferably a temperature is maintained which lies above thetemperature of condensation of the hydrochloric acid, that is, at normalpressure, above 110 C.

In case it is desired to evaporate aqueous hydrochloric acid whichcontains small quantie ties of sulphuric acid, in an apparatusconsisting of copper, it is recommended to add to the phosphoric acidsalts of metals soluble in excess phosphoric acid which with thesulphuric acid form deposits diilicultly soluble or insoluble inphosphoric acid. By the precipitation of the sulphuric acid theconcentration of the latter in the evaporating apparatus and consequentcorroding actions are avoided. In particular soluble alkaline earthmetal salts such for example as barium or calcium chloride and the likeare suitable for precipitating the sulphuric acid.

Since the precipitated alkaline earth metal sulphate forms a suspensionin the phosphoric acid which suspension together with the copperchloride resulting from a certain not entirely avoidable corrosion ofthe apparatus, and likewise forming a suspension in the phosphoric acid,and other impurities contained in the hydrochloric acid. would graduallyrender the phos- .phoric acid unsuitable for carrying out the process,it is recommended to remove the deposit formed in the phosphoric acid ina suitable manner, for example by filtration or centrifuging. and thiscan be effected periodically or also continuously during the operation,by continuous removal of a small quantity of phosphoric acid which isreturned after having been cleansed.

The evaporation can be carried out in various ways. For example theprocess may be a discontinuous one. in which a mixture of D1 phoric acidand hydrochloric acid is first introduced into the still of theevaporating apparatus and then subjected to evaporation. Preferably,however, a continuous method is employed in which concentratedphosphoric acid, which-may if desired contain some barium chloride, isintroduced into the-evaporating vessel, this is then heated and thenwith continued vigorous heating the aqueous hydrochloric acid.preferably prewarmed. is caused tofiow into the hot phosphoric acid. Theconcentration ofthe phosphoric acid employed can be suitably selectedwithin wide limits, with corresponding regulation of the heat-- ing andcorresponding regulation of the quantity of hydrochloric acid flowingin, for the final concentration adjusts itself automatically in theaosmvc course of the'evaporation in dependence on the heatingtemperature employed.

In the following the manner of carrying out the new process and theapparatus employed will be explained with reference to one embodiment byway of example and it is expressly to be observed that the presentinvention is not limited to the proportions and dimensions, temperaturesand the like mentioned, but that modifications such as would occur toanyone skilled in the art may of course be made.

In the embodiment to be described the actual evaporation chamberconsists of a horizontal cylindrical vessel of copper having a length ofabout 3.5 metres and a diameter of about 1.4 metres. The bottom of thevessel is provided with steam tubes which have an external diameter ofabout 50 mm. and a length of about 80 metres.

' On the outsideit is provided with good heat insulation. The upperparts of the evaporating g0 chamber which are not covered or washed overby the liquid are provided oh the outside with. iron steam tubes whichwith a view to reasonable transfer of heat liedirectly on the copperwall. As has already been explained they are intended g, to heat theupper part of the boiler in such manner that the condensation ofhydrochloric acid vapour is prevented. The upper part of the evaporatingvessel together with the heatin tubes are likewise heat insulated. Theevaporating vessel is half filled with phosphoric acid of about 80% towhich are added about 0.5 to 2% of barium chloride. gipce during theoperation the strength of the acid is adjustable by variation of thequantity of heating steam or the quantity of hydrochloric acid flowingin as may be desired, it is also possible to start with a weaker orstronger acid. The contents of the boiler are then heated to about 150centigrade after which with further heating about 1000 kilos of 17%hydrochloric acid are allowed to flow hourly into the phosphoric acidpreferably distributed at diiferent places. After attaining thecondition of equilibrium the vapours drawn oil. show the samecomposition as the hydrochloric acid added. During the evaporation about50 litres of phosphoric acid are continuously drawn out of theevaporator hourly by means of a small pump and forced through a filterconsisting of fine copper wire mesh which separates the impuritiescontained in the phosphoric acid. The filtered phosphoric acid fiowsback continuously to the evaporator. with the quantity of phosphoricacid given the evaporation of aqueous hydrochloric acid can be carriedout in a" continuous manner for a very long time without any renewal ofthe phosphoric acid being necessary.

what I claim is:

1. A. method evaporating aqueous hydrochloric acid consisting incarrying out the evaporation in an apparatus made of copper in thepresence of. phosphoric acid.

2. A method of evaporating aqueous hydrochloric acid consisting incarrying out the evaporation in an apparatus made of copper in thepresence of phosphoric acid of over 60%.

3. A method of evaporating aqueous hydrochloric acid consisting incarrying out the evaporation in an apparatus made of copper in thepresence of phosphoric acid the parts of the apparatus not touched bythe phosphoric acid being heated to a'temperature lying above thetemperature of condensation for aqueous hydrochloric acid.

4. A method of evaporating aqueous hydrochloric acid, containing and asan impurity consisting in carrying out the evaporation in an apparatusmade of copper in the presence of of phosphoric acid to which salts ofmetals are added which are soluble in excess phosphoric acid and whichform with sulphuric acid salts difilcultly soluble in phosphoric acid.

5. A method of evaporating aqueous hydrochloric acid containing sulfuricacid as an impurity consisting in carrying out the evaporation in anapparatus made of copper in the presence of phosphoric acid to whichalkaline earth metal salts which are soluble in excess phosphoric acidare added.

6. A method of evaporating aqueous hydrochloric acid containing sulfuricacid as an impurity consisting in carrying out the evaporation in anapparatus made of copper in the presence of phosphoric acid to whichalkaline earth metal chlorides are added.

7. A method of evaporating aqueous hydrochloric acid consisting incarrying out the evaporation in 'a continuous process in an apparatusmade of copper inthe presence of phosphoric acid.

8. A method of evaporating aqueous hydrochloric acid containing sulfuricacid as an impurity consisting in carrying out the evaporation in anapparatus made of copper in the presence of phosphoric acid to which areadded salts of metals which are soluble in excess phosphoric acid andwhich with sulphuric acid form salts which are diflicultly soluble inphosphoric acid the deposits which form and which are difllcultlysoluble in phosphoric acid being removed by filtration.

9. A method of evaporating aqueous hydrochloric acid containing sulfuricacid as an impurity consisting in carrying out the evaporation in anapparatus made of copper in the presence of phosphoric acid to which areadded salts of metals which are soluble in excess phosphoric acid andwhich with sulphuric acid form salts which are dlfilcultly soluble inphosphoric acid, the deposits which form and which are diflicultlysoluble 1 in phosphoric acid being continuously removed by filtration.

KLAUS RASCHIG.

CERTIFICATE OF CORRECTION.

Patent No. 2,051,179. February 1a, 1936.

KLAUS RASCHIG.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,second column, line 59, claim 1, after the word "method" insert of; andline 75,

claim 4, for "and" read acid; and that the said Letters Patent should beread with these corrections therein that the same may conform to therecord of the case in the Patent Office.

Signed and sealed this 23rd day June, A. D. 1936.

Leslie Frazer iSeal) Y Acting Commissioner of Patents.

